This invention relates generally to the preparation of uniform mixed metal oxides, which are important in certain advanced ceramics applications. One current topic of great interest is the preparation of copper oxide-based superconducting materials, particularly a composition containing yttrium, barium, and copper. Chu and coworkers reported the discovery of a new material based on the starting composition Y.sub.1.2 Ba.sub.0.6 CuO.sub.y with a superconducting temperature well above 90.degree. K. See, Physics Review Letters, 58, 908 (1987). However, only a fraction of this sample was actually superconducting, and the superconducting fraction was identified as an oxygen defect perovskite corresponding to the composition YBa.sub.2 Cu.sub.3 O.sub.y.
The synthesis of these new superconductors follows standard methods previously described for perovskite oxides. This involves intimately mixing the oxide or the carbonate powders and calcining at temperatures ranging from 900.degree. to 1000.degree. C. For example, to obtain YBa.sub.2 Cu.sub.3 O.sub.y, ultrapure Y.sub.2 O.sub.3, BaCO.sub.3 and CuO powders are mixed in a ball mill in an atomic ratio of Y/Ba/Cu of 1/2/3. After extensive milling, the powder is heated in an alumina boat at 950.degree. C. under flowing oxygen for 12 hours. The resulting black powder is remilled and heated again under similar conditions to give the desired phase.
Reaction of the components occurs by solid state diffusion of the yttrium, copper and barium ions at high temperature. Since the mobility of these ions is limited to relatively short distances, extensive milling is required to give as homogeneous a mixture as possible. However, achieving a homogeneous mixture of the material on a microscopic scale is extremely difficult. On a commercial scale the milling of metal compound powders could be expected to be cumbersome, if not practically impossible, and consequently, another method capable of providing homogeneous mixtures would be desirable.
Homogeneity of superconducting materials is very important because the maximum amount of current, i.e. critical current, which the superconducting material can conduct is a function of the homogeneity and microstructure of the material. Additionally, the homogeneity and the microstructure of these superconducting materials affects the ability of these materials to be fabricated into useful structures such as wires, coatings or tapes.
In copending application, U.S. Ser. No 07/306,233 the coprecipitation of metal oxide precursors from aqueous solutions of their salts by addition of oxalic acid was shown to make possible the preparation of superconducting compositions. Such a method could be used on a commercial scale.
More recently, it has been found that some of the barium remains in the aqueous solution during the precipitation of the metal oxalates when the nitrates are used as starting materials. This may be attributed to the formation of nitric acid as the metal nitrate is precipitated as the metal oxalate. As a result the ratio of the metals initially dissolved is not identical with that in the precipitated mixture. In addition, the chemical composition of the precipitates is not perfectly uniform and barium tends to be segregated.
Peng Ding-Kun et al. show that in aqueous solutions the pH should be controlled and acetate salts should be used to control the composition of the precipitate. They report that barium in particular remains in solution rather than being fully precipitated.
Moure et al. in Br. Ceram. Proc., 40 (Supercond. Ceram.), 237-42, discuss the product obtained when Y, Ba, Cu nitrates were precipitated by oxalic acid dissolved in ethanol. The nitrate precursors were said to have been dissolved in an ethanol-water solution, presumably because the nitrates are not soluble in ethanol, but require the presence of water.
Consequently, we have sought improved methods which provide superior superconducting compositions. However, these methods are not limited to preparing superconducting compositions but are more generally applicable to those situations where extremely uniform mixtures of metal oxides are wanted.
The instant invention provides an improved method of manufacturing a uniform mixed metal oxide such as a superconducting material by coprecipitating insoluble solids of the desired components from a particular medium, then drying and calcining said solids. This process provides materials that are intimately mixed at room temperature without having to rely on milling or high temperature diffusion techniques. It has been found that by proper selection of the initial metal precursors and the precipitating medium that the resulting precipitate decomposes to a metal oxide mixture at a temperature up to 100.degree. C. lower than obtained by prior art precipitation methods, which indicates that a more uniform mixed metal oxide has been produced. Where a superconducting compound is sought, a greater fraction of the precipitate is converted to a superconducting material.